Engine: geometry.datum.point; planes and axes through datum points; datum edits and cascading delete

A datum point by coordinates, at a mesh vertex, a fraction along a
design edge, at a face's centre, or where a datum axis crosses a datum
plane. Datum planes take three datum points or a normal to a datum axis;
axes take two datum points. geometry.datum.edit, .axis.edit and
.point.edit redefine a datum of their kind (partitions cut again);
geometry.datum.delete with cascade also deletes the partitions that cut
by it. The ribbon shows Datum Plane, Datum Axis, Datum Point and
Partition, each with its own icon; edit and delete leave the ribbon
(still commands, claimed by their entries).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

assets/icons/src/geometry.datum.point.json

@@ -0,0 +1 @@
+{"shapes": [{"d": "M3 16 L9 10 L21 10 L15 16 Z", "fill": "soft", "stroke": "line", "width": 1.25}, {"d": "M12 3 L12 11 M8 7 L16 7", "stroke": "accent", "width": 2}, {"circle": [12, 7, 1.8], "fill": "warm", "stroke": null}, {"d": "M12 12.5 L12 20", "stroke": "line", "dash": [1.5, 2]}]}